cgroppi, in answer to your question on whether a 285 will fit on the stock 18x9 out back. The official answer from Hoosier is "not recommended". The 275's will fit a 9"rim no problem. The 285's really need an extra 1" of rim width. However as you've pointed out other cars at AX Nationals do run the biggest tire possible. IMHO the wider tire's will balloon the sidewall, allowing more flex than the 275 which will reduce feel & reduce contact patch. If i remember years ago tirerack & grassroots motorsport magazine did an article on this very subject. But you pays your money & makes your choice. Let us know how you get on I'm very interested in your real world experience.
Tire Size. TreadWidth. Recom Rim. Measured Rim
P275/35ZR18 10.3" 9-11" 9.5"
P285/30ZR18 10.9" 10-11" 10.0"
Data from Hoosier's site
https://www.hoosiertire.com/rrtire.htm